Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a test gas, which passes through the smallest of leaks in a sealed test piece. After passing through a leak, a test sample containing helium is drawn into a leak detection instrument and is measured. In the instrument, a mass spectrometer tube detects and measures the helium. The input test sample is ionized and mass analyzed by the spectrometer tube in order to separate the helium component. In one approach, a test piece is pressurized with helium. A sniffer probe connected to the test port of the leak detector is moved around the exterior of the test piece. Helium passes through leaks in the test piece, is drawn into the probe and is measured by the leak detector. In another approach, the interior of the test piece is coupled to the test port of the leak detector and is evacuated. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
One of the difficulties associated with helium mass spectrometer leak detection is that the inlet of the mass spectrometer tube must be maintained at a relatively low pressure, typically 2×10−4 Torr. In a so-called conventional leak detector, the test port, which is connected to the test piece or to the sniffer probe, must be maintained at relatively low pressure. Thus, the vacuum pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, it may be difficult or impossible to reach the required pressure level. If the required pressure level can be reached, the pumping cycle is lengthy.
Techniques have been proposed in the prior art to overcome this difficulty. A counterflow leak detector is disclosed in U.S. Pat. No. 3,690,151, issued Sep. 12, 1972 to Briggs, utilizes a technique of a reverse flow of helium through a diffusion pump to the mass spectrometer. The leak detector test port can be operated at the pressure of the diffusion pump foreline. A similar approach uses reverse flow of helium through a turbomolecular pump. U.S. Pat. No. 4,735,084, issued Apr. 5, 1988, to Fruzzetti, discloses a technique for gross leak detection wherein the test gas is passed in reverse direction through one or two stages of a mechanical vacuum pump. These techniques have permitted the test port pressure to be higher than for conventional leak detectors. Nonetheless, reaching the higher test port pressure can be difficult when testing large volumes, dirty parts, or parts with large leaks.
French Patent No. 1,181,312, published on Jun. 15, 1959, discloses a helium leak detector that uses a heated silica membrane to selectively pass helium and hydrogen. A getter is provided to capture the hydrogen, and an ionization gauge is used to measure the helium pressure. European Patent Application No. 0352371, published Jan. 31, 1990, discloses a helium leak detector including an ion pump connected to a probe in the form of a silica glass capillary tube. The silica glass tube is heated to a temperature between 300° C. (degrees Celsius) and 900° C. and thereby becomes permeable to helium. U.S. Pat. No. 5,325,708, issued Jul. 5, 1994, to DeSimon, discloses a helium detecting unit using a quartz capillary membrane, a filament for heating the membrane and an ion pump. U.S. Pat. No. 5,661,229, issued Aug. 26, 1997, to Bohm et al., discloses a leak detector with a polymer or heated quartz window for selectively passing helium to a gas-consuming vacuum gauge.
Leak detection techniques, which utilize a permeable membrane permit the test gas sensor to operate at a different pressure from the leak detector inlet. However, prior art membranes have had low permeance at room temperature and have typically required heating to a high temperature to increase permeance. The heated membrane requires a controlled heating source, thereby increasing the cost and complexity of the unit. In addition, the permeance of prior art membranes has had a relatively large temperature coefficient. Thus, the accuracy of leak detection depends in part on the accuracy with which the temperature of the membrane is controlled.
Leak detection techniques which utilize a permeable membrane are also vulnerable to too much helium reaching the ionization gauge, ion pump, or gas-consuming vacuum gauge. If a large leak is encountered, the resulting helium partial pressure outside the membrane drives a large number of helium atoms into the sensor. Once there, these sensors cannot rapidly bury all these helium atoms. This can render the leak detector inoperable, or with reduced sensitivity, for several minutes.
Accordingly, there is a need for improved methods and apparatus for leak detection.